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MDEV-16934 Query with very large IN clause lists runs slowly

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This patch introduces support for the system variable eq_range_index_dive_limit
that existed in MySQL starting from 5.6. The variable sets a limit for
index dives into equality ranges. Index dives are performed by optimizer
to estimate the number of rows in range scans. Index dives usually provide
good estimate but they are pretty expensive. To estimate the number of rows
in equality ranges statistical data on indexes can be employed. Its usage gives
not so good estimates but it's cheap. So if the number of equality dives
required by an index scan exceeds the set limit no dives for equality
ranges are performed by the optimizer for this index.

As the new system variable is introduced in a stable version the default
value for it is set to a special value meaning there is no limit for the number
of index dives performed by the optimizer.

The patch partially uses the MySQL code for WL 5957
'Statistics-based Range optimization for many ranges'.
This commit is contained in:
Igor Babaev
2018-08-17 14:27:42 -07:00
parent d6f7fd6016
commit 4eac5df3fc
13 changed files with 239 additions and 19 deletions
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27
sql/opt_range.cc
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@ -14617,6 +14617,33 @@ void QUICK_GROUP_MIN_MAX_SELECT::add_keys_and_lengths(String *key_names,
}
/* Check whether the number for equality ranges exceeds the set threshold */
bool eq_ranges_exceeds_limit(RANGE_SEQ_IF *seq, void *seq_init_param)
{
KEY_MULTI_RANGE range;
range_seq_t seq_it;
uint count = 0;
PARAM *param= ((SEL_ARG_RANGE_SEQ*) seq_init_param)->param;
uint limit= param->thd->variables.eq_range_index_dive_limit;
if (limit == 0)
{
/* 'Statistics instead of index dives' feature is turned off */
return false;
}
seq_it= seq->init(seq_init_param, 0, 0);
while (!seq->next(seq_it, &range))
{
if ((range.range_flag & EQ_RANGE) && !(range.range_flag & NULL_RANGE))
{
if (++count >= limit)
return true;
}
}
return false;
}
#ifndef DBUG_OFF
static void print_sel_tree(PARAM *param, SEL_TREE *tree, key_map *tree_map,